For the subjective refraction determination, examination devices such as measuring spectacles or phoropters are known, in which, for example, spherical or cylindrical testing lenses are pivoted in front of a subject's eye to determine the refractive error of the subject on the basis of his (or her) statements.
In the subjective refraction determination in accordance with national and international standards, defined vision symbols, also known as optotypes, are always used. The optotypes are shown in black on a white background with a high contrast. The stroke width (⅕ of the type size) of the Landolt ring, which is accepted as a standardized DIN vision symbol, is such that, in a row intended for visual acuity 1, it appears to the eye at an angle of 1 arc minute. Further vision symbols are the tumbling E, which is frequently used to examine children. In examinations of near visual acuity, a letter chart is frequently used, for example what are known as Nieden reading charts. The optotypes are represented individually or in rows of 5 or 10 optotypes.
For the refraction determination, optotypes of different sizes are successively shown to the subject at a defined distance. For this, the subject sits at a predetermined distance from the displayed optotypes. The defects of the eyes are determined by approaching the threshold of the resolution capability of the subject.
Document U.S. Pat. No. 6,325,513 describes as a disadvantage in the related art the fact that the subject, in the case of such a measurement, does not assume a relaxed posture and the natural visual impression is falsified. For this reason, the document proposes a type of compact measuring spectacles, in which the symbols for refraction determination are projected onto the retina. With the solution proposed therein, superposition of produced measurement images with the environment is made possible with a comfortable, relaxed sitting posture. The subjective refraction determination is affected with reference to the optotypes.
Document U.S. Pat. No. 5,026,151 A discloses an apparatus for binocular eyesight testing having a display apparatus, on which eyesight test symbols may be represented for the left and the right eye in alternation. A controllable closure device is provided herefor, which is controlled such that some of the represented eyesight test symbols are shown only to the left eye, some of the represented eyesight test symbols are shown only to the right eye, and some of the eyesight test symbols are shown to both eyes. This makes possible binocular eyesight testing. The display apparatus can be a television monitor having a sufficiently high frame rate. However, a television monitor typically does not have a sufficiently high luminance to represent the separate vision symbols for each individual eye at the required luminance (for example 250 cd/m2).